Efforts to make portable useful devices and electronic systems that are already sufficiently miniaturized by virtue of modern fabrication techniques often clash with the difficulty of finding portable power sources capable of ensuring a prolonged service, that is of power packs of suitably high energy/volume ratio. In fact, notwithstanding miniaturization and a constant reduction of current absorption of modern integrated electronic systems that reach ever higher levels of compactness and reduction of size of integrated structures the accompanying increase of the number of functions of these portable devices poses heavy requirements on rechargeable batteries as commonly used in these portable devices.
In view of the limits of the obtainable energy/volume ratio even for the most advanced commercially available rechargeable batteries, there is a growing interest on primary energy converters among which in particular on fuel cells capable of transforming chemical oxidation energy of a fuel (typically hydrogen or methanol or other oxidable compound in gaseous form of in solution), into electrical energy. Fuel cells include catalytic electrodes permeable by the fluid reagent, separated by an electrolyte, generally a solid polymer electrolyte constituted by a film of ion exchange resin, typically for protons (H+), which besides constituting the medium (electrolyte) through which an ionic current may flow, also ensure a physical separation of the fuel that is fed to the negative porous catalytic electrode (anode) from the oxygen (air or oxygen in a mixture or pure oxygen) fed to the positive porous catalytic positive counterelectrode (cathode) of the cell.
Fuel cells offer an energy/volume ratio much greater than most advanced rechargeable batteries and its operating life is theoretically unlimited as long as there is availability of fuel and suitable fuels may be stored in large quantity in relatively small and lightweight reservoirs, even at superatmospheric pressure.
The general structure of fuel cell for large power applications such as for electrical vehicles or designed for a context of capillary use of ducted hydrogen as energy vector (hydrogen economy) is well known and sufficiently described in literature. More specifically, important studies are being conducted for realizing fuel cells in micrometric scale directly on monocrystalline silicon, by exploiting modern techniques of silicon micromachining (MEMS) through chemical/electrochemical preferential etchings of specifically doped regions of the crystal followed by oxidation of residual porous silicon and leaching of the oxidized residual silicon, that have been developed for realizing sensors, actuators, transducers and passive electrical components integratable on the same chip on which is integrated the circuit or electronic subsystem using the sensor, actuator or the component formed by micromachining the silicon crystal.
U.S. Pat. Nos. 6,541,149-6,811,916-6,558,770-6,641,948-5,316,869-6,627,342-6,740,444-6,506,513-6,589,682-6,610,433, the published US Patent Applications Nos. 2003/0022052A1, 2003/0096146A1, 2002/0020053A1, 2003/0134172A1, 2002/0041991A1, 2003/0003347A1, 2004/0058153A1, the documents WO0069007, DE19914681A, WO 0045457, DE19757320A, JP07201348 and EP-A-1258937, provide a survey of known architectures of microfuel cells formed on silicon. As may be observed, the approach has so far been based on the realization on a silicon substrate of a semicell structure provided with relative inlet, distributing ducts of the relative fluid reagent to the porous catalytic electrode structure of the semicell and eventual outlet or vent.
The two semicells made on distinct dies of the silicon crystal, in a substantially specular manner, are thereafter joined by interposing between them, over the whole active area of the cell, an ion exchange resin separator, over the opposite surface of which may have already been incorporated an intimately adherent layer of particles of a catalytic electrodic material, thus bringing therefore the active surfaces of the two porous electrodic structures in contact with the ion exchange resin of the separating membrane sandwiched between the two silicon semicell structures.
The constitution of the fuel cell with two distinct dices of silicon through wafer bonding techniques greatly complicates the electrical interconnectivity of the electrodes of a cell with electrodes of other cells that may be formed on the same device to achieve, through series-parallel interconnections of cells, an electric current source at a certain voltage (i.e. an integral multiple of the elementary cell voltage), as well as between the so-constituted electrical source and the integrated circuitry to be powered, which may be realized on a separate chip or even on the same chip on which one or more semicell structures are formed.
Another aspect of known architectures of silicon fuel cells is the need of forming metal grid current distributors over and in contact with the respective porous catalytic electrode layers intimately formed in contact with the ion exchange resin of which the relative half-cell electrode reaction of oxidation (at the anode of the cell and of reduction of the cathode at the cell) takes place. In fact, the porous high specific surface area of the catalytic electrode particles in contact with the solid electrolyte constituted by the ion exchange resin of the separating membrane, and the relative thinness of these electrode layers, determine a non-negligible electrical resistance to electronic current flow on the plane of the layer, that often is incompatible with the need of realizing an active electrode area sufficiently large to produce the desired current, considering the limit to the current density on the active cell area that may be achieved at an acceptable voltage.
In realizing multi cell bipolar stack of fuel cells to generate a battery voltage multiple of the elementary cell voltage, the classic approach is to realize bipolar electrode septa (plates), having inlet and eventually outlet ports for feeding respective reagent fluids to the porous electrode structures (eventually comprising even a superficial layer of catalytic material) formed on the two opposite surfaces of the bipolar conductive silicon plate to be eventually stacked interposing between adjacent bipolar (electrodes) plates membranes of ion exchange resin that may already be provided with a bonded layer of particles of catalytic materials over the active cell area on the opposite surfaces of the ion exchange membrane that are contacted by respective porous electrode structures of the conducting bipolar plates (re: US 2004/0185323A1 and U.S. Pat. No. 6,589,682).
Negative aspects deriving on the way the permeable porous electrode structures of the cells can be formed, are their scarce mechanical sturdiness that is poorly suited for a sandwich like clamping of the ion exchange resin membrane between opposing surfaces of two silicon plates. Although during deposition of the catalytic metal on porous silicon electrode structures the deposited metal joins to the definition edges of previously formed metal lines of a current collecting grid, already exploited as a hard mask for forming the porous silicon electrode compartment, the composite structure remains mechanically weak.
On account of the current density limits per unit of active area of fuel cells determined by technological limitations of ensuring a sufficient mass transfer of reagents to the active sites of electrochemical half-cell reactions, the area of monocrystalline silicon that is required for generating a certain maximum nominal current that can be absorbed by an electrical load at a certain voltage, represents an important factor for the most cost-efficient exploitation of silicon area for forming the fuel cells. Its significant improvement would permit to significantly lower the cost of these devices.